High strength alloy for use at elevated temperatures



United States atent O HIGH STRENGTH ALLOY FOR USE AT ELEVATED TEMPERATURES Peter J. Clemm, Ballston Lake, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 6, 1958, Ser. No. 719,495

9 Claims. (Cl. 75-171) This application is a continuation-in-part of my application Serial No. 459,831, filed October 1, 1954, now abandoned, and assigned to the same assignee as the present invention.

The present invention relates to alloys which have high strength at temperatures up to and exceeding 1650 F. and wrought articles made from such an alloy.

The development of the gas turbine and similar apparatus has created a demand for materials having higher strength and greater stability at elevated temperatures than the high temperature alloys currently available. Certain components of such apparatus must be capable of withstanding very high stresses over long periods of time in highly corrosive atmospheres at continuous temperatures of the order of l500 F. or more. Many of the presently available alloys having high strength at elevated temperatures are casting alloys which can be forged, rolled or machined only with great difficulty. Since the apparatus components made from these alloys require close manufacturing tolerances, an alloy which can only be cast poses many difficulties only one of which is the difiiculty of maintaining a high degree of uniformity of quality in known precision casting procedures. A wrought alloy having the necessary high temperature strength would not only solve many manufacturing problems, but also provide a better and less expensive product. The term wrought alloy is used to describe alloys which may be fabricated into finished articles by forging or rolling to distinguish from alloys which, because of their inherent resistance to deformation without fracture, are only suitable for casting.

Many of the previously known wrought alloys have exhibited high initial strength at elevated temperatures but, after a relatively short period of time under these conditions, have lost certain desirable properties, particularly strength and ductility. This loss of strength and ductility as a function of time and temperature has been attributed in part to an inherent physical instability of these alloys. It has been observed that these undesirable changes and mechanical properties are nearly always accompanied by significant changes in the microstructure of the alloys. These changes are usually a significant increase of grain size or the precipitation of intermetallic compounds and phases, particularly at the grain bound aries or, frequently, both. Therefore, it appears desirable that an alloy which is to retain high strength over long periods of time at high temperature be resistant to grain growth and intergranular precipitation at elevated temperatures.

A further problem encountered in the operation of such devices is chemical instability of materials exposed to corrosive atmospheres at the operating temperature under high stress. Inasmuch as one of the applications contemplated for my alloy is gas turbine buckets and blades, such an alloy should be resistant to a severely oxidizing atmosphere at elevated temperatures and should not excessively sp-all or scale.

The principal object of this invention is the provision of an alloy which has high strength at elevated temperatures, which is both chemically and physically stable under high temperature and stress and which may be readily fabricated by the usual methods of forging, rolling and machining.

A further object of this invention is the provision of wrought articles which, at 1650" R, will withstand stresses of substantially the same magnitude as similar articles made of known alloys withstand at 1500 F.

Other and specifically different objects of my invention will become apparent to those skilled in the art from the following disclosure.

Many attempts have been made to produce a wrought alloy having good mechanical properties at elevated temperatures of the order of 1500 to 1600 F. Two accepted standards of comparison for judging such alloys are the stress rupture properties and fatigue strength. Rupture strength is usually expressed as the constant load in pounds per square inch (p.s.i.) necessary to break a test specimen of the alloy under consideration in tension in a given length of time at a given constant temperature. The fatigue strength of an alloy as herein used is expressed as the limiting stress in p.s.i. which when applied cyclically will cause failure in 100,000,000 cycles at a given constant temperature.

One of the better commercial wrought alloys currently available, hereinafter referred to as the reference alloy for purposes of comparison, is composed of approximately 44 percent, by weight, cobalt, 20 percent nickel, 20 percent chromium, 4 percent tungsten, 4 percent molybdenum, 4 percent niobium, the balance being iron. On an atomic percent basis, this alloy has a composition of approximately 45.3 percent cobalt, 20.7 percent nickel, 233 percent chromium, 1.3 percent tungsten,

. 2.5 percent molybdenum, 2.6 percent niobium, balance iron. The fatigue strength of this commercial alloy for 10 cycles tested at 1500 F. is about 35,500 p.s.i. The stress rupture strength of this alloy for hours at 1500 F. averages about 23,600 p.s.i., for 1000 hours at 1500 F. about 18,000 p.s.i. or less, and for 100 hours at 1650 F. about 12,500 p.s.i. and for 1000 hours at 1650 F. about 7,800 p.s.i., minor variations occurring between different heats or batches, depending upon the amount and character of the impurities, the heat treatment, and upon how closely the actual composition compares with that cited above. The fatigue and stress rupture strengths given for this alloy represent test results obtained from test specimens which have been forged or otherwise wrought and ground from ingots and therefore are representative of the wrought properties of this alloy. A comparison of the rupture strengths of this material at the two temperatures reveals a very significant and undesirable loss of strength between 1500 F. and 1650 F. The importance of this loss of strength in this temperature range may beillustrated by stating that if the operating temperature of the turbine buckets in any of the aircraft turbojet engines now in use could be raised 100 F. without any attendant loss of strength, the thrust of these engines would be increased about 10 percent.

The alloy of my invention is composed of cobalt, nickel, chromium, iron and carbon with hardening and strengthening additions of titanium, tungsten, molybdenum, niobium and tantalum, plus a small but relatively critical amount of boron.

The titanium content of this alloy is most effective as a strengthening agent when precipitated as a finely dispersed intermetallic compound by appropriate heat treatment. For example, in the alloy of my invention titanium is first put into solid solution by heating the alloy to an appropriately high temperature and then aging at a lower temperature. During the aging anneal an intermetallic compound such as Ni Ti, for example, is precipitated. If the effective titanium content of this alloy is reduced due to substantial portions of the titanium being tied up as titanium oxide, nitride, carbide or like compounds, present as inclusions, less of the titanium can form the hardening and strengthening constituent upon heat treatment and therefore a weaker alloy will result. At the high temperatures necessary to melt the various constituents of alloys of this type, titanium has an exceedingly high rate of reaction with certain of the gases of'the atmosphere rendering it extremely diflicult to alloy more than about 2.5 percent, by weight, of titanium in an air melt. Therefore, in order to prevent the dissipation of any substantial part of the titanium during melting by reaction with the atmosphere, it is preferred that a vacuum melting process equivalent to that described in the US. patent to J. D. Nisbet 2,564,498 (assigned to the assignee of the present application) be employed. This patent discloses a melting schedule in which the metal is melted in a vacuum furnace subjected to a first deoxidation treatment using hydrogen, carbon or the like, a final deoxidation treatment by the addition of titanium or zirconium and is then cast. Since the limitation on the content of the strengthening agent titanium composed by air melting is not existent under the cited vacuum melting process, my alloy may contain more than 3 percent, by weight, of titanium.

In order to correlate and evaluate the effects of the various additions to the basic alloy, compositions of the alloy of my invention are set forth in atomic percents since the elements comprising the variants have, of course,

of 95.95 while one atom of tungsten has a relative weight of 183.92 and yet with reference to the other materials of the alloy, each of the atoms has otherwise substantially the same strengthening effect as the other. It will there- 5 fore be understood that hereafter in the specification and claims, percent means atomic percent and not weight percent.

My invention comprises an alloy having a composition within the range of up to 2 percent iron, 35 to 40 percent cobalt, 25 to 30 percent nickel, 18 to 25 percent chromium, 1.5 to 5 percent tungsten, 0 to 3.5 percent molybdenum, 0 to 3.5 percent tantalum, 0 to 1.5 percent niobium, 4 to 7.5 percent titanium, and 0.5 to 1.5 percent carbon, 0.01 to 0.4 percent boron. Particularly strong alloys in this range are, for example, composed of 37.7 percent cobalt, 28.2 percent nickel, 23.2 percent chromium, 2.35 percent tungsten, 2.44 percent molybdenum, 4.75 percent titanium, 1.15 percent carbon and 0.28 percent boron; 37.6 percent cobalt, 28.2 percent nickel, 22.6 percent chromium, 3.86 percent tungsten, 0.83 percent tantalum, 4.6 percent titanium, 0.98 percent carbon, 0.056 percent boron, balance iron; 37.1 percent cobalt, 27.6 percent nickel, 22.2 percent chromium, 1.88 percent tungsten, 2.02 percent molybdenum, 1.11 percent tantalum, 5.95 percent titanium, 0.7 percent carbon, 0.012 percent boron, balance iron; and 37.2 percent cobalt, 27.8 percent nickel, 22.4 percent chromium, 2.01 percent tungsten, 2.12 percent molybdenum, 1.03 percent niobium, 4.98 percent titanium, 1.04 percent carbon, 0.054 percent boron, balance iron, as will be more particularly disclosed later. I

This alloy is of austenitic constituency and has excellent corrosion resistance at elevated temperatures. In order to better illustrate and disclose the many advantages Which my invention oifers the art of high temperature, high strength alloys and wrought articles for use under high temperature and high stress, various tests were performed. A large number of heats or batches of my alloy were made and so tested.

Specifically, test specimens of my alloy were prepared by hot forging bars from cast ingots, swaging and grinding the bars into conventional test specimen configurations which were tested according to accepted practices for stress rupture properties, fatigue strength, tensile strength and ductility. The tensile strength of these alloys are expressed in pounds per square inch and represent the short time stress required to break a test specimen in tension. Ductility is expressed in percent elongation in one inch of a test specimen which has been broken in tension and stress rupture.

In order to illustrate the effect of varying the chemical constituency of my alloy upon its mechanical properties, the following examples may be compared.

TABLE I Heat Fe C0 Ni Cr W M0 Ta Nb Ti C B different atomic weights and react with the other ele ments of the alloys on an atom-to-atom basis. For example, one atom of molybdenum has a relative weight The 100 and 1000 hour stress rupture strength of these alloys expressed in pounds per square inch is given in the following table for various testing temperatures.

Age Test Tensile Percent Stress Life Percent Heat F.) F.) Strength Elong. (p.s.i (hours) Elong. (p.s.i.) (1" ga.) (1 ga.)

1, 500 R.T. 180,000 24. 4 A 1, 500 1, 500 109, 800 10.1 1, 650 1, 500 30, 000 325 21.0 1, 600 R.T. E 1, 600 1, 500 33.0 1, 700 1, 500 23.0 1, 500 1, 500 13. ,7 1, 500 1, 650 29. 5 1, 600 R T. 5. l ,H 1, 600 1,500 92, 800 16. 9 30,000 305 15. 3 ,600 1, 650 000 71 11.6 1, 700 1, 500 30, 000 1, 103 7. 5 1, 700 1, 650 20, 000 53 16. 5 1, 500 1, 500 30, 000 406 27. 5 1, 500 1, 650 20, 000 151 16. 7 1, 000 R.'I. 169,800 5. 5 I 600 l, 500 100, 400 19. 2 30, 000 1, 051 14. 3 ,500 1, 650 335 9. 7 1, 700 1, 500 30, 000 1, 483 16. 2 1, 700 1,650 20, 000 410 12. 6 1,650 R.T. 152,100 19. 1, 650 1, 500 75, 300 12.1 30,000 445 39. 0 J 1, 650 1, 600 20, 000 681 22. 0 1, 650 1, 650 43, 900 0 1, 650 l, 700 15,000 195 26.0 1, 650 R.T. 123, 200 7. 9 1, 500 85, 10 23. 3 30, 000 371 26.0 R 1, 600 20, 000 562 25. 0 1,650 47,200 62.0 1, 0 15,000 220 33.0 1,600 1, 650 ,000 294 25. 0 1, 675 R11. 136, 300 3. 0 N 1, 675 1, 650 58, 800 36. 2 20, 000 216 19. 0 1,750 R.T. 131,900 4.3 1, 750 1, 650 57, 600 36. 6 20, 000 236 16.0 1, 600 1, 650 .000 363 17.0 1, 675 R11. 156, 000 6. 3 P 1, 675 1, 650 61, 000 36. 7 20, 000 438 11. 0 1,675 1,700 20,000 200 8.0 1 750 RT. 156, 000 6. 6 1, 750 1, 650 200 44. 4 20, 000 483 10.1

In the following table typical test results are set forth to illustrate the fatigue strength of two of these alloys.

TABLE II Stress rupture strength [Pounds per square inch] 1,500 F. 1,600 F. 1,650 F. 1.700 F.

Heat

100 hr. 1,000 100 hr. 1,000 10011r. 1,000 100111. 1000 hr. hr. hr. hr.

In the following table a few representative heats were TABLE IV selected in order to better illustrate specific stress rupture u tre th data as well as to dlsclose how the tensile strength of Fang e 8 ng these alloys varies with varying compositions and heat Test qtress Minions of treatments. All of the following test specimens have Heat (F.) (11st) cycles to been solution treated for four hours at 2150 F. and aged for 24 hours at the indicated temperatures. The symbol 1 0 0 8 RT. indicates a test performed at room temperature. 2253 88 f 83 J 1, 500 50, 000 0. s 1,500 ,0 0.157 35 152 35888 :15 TABLE III 11500 551000 0.12 K 1, 500 45,000 0. 69 1, 500 45, 000 6. 47 Mechamcal properties 1 Specimen unbroken.

It will be observed from Table I that heat A is very similar in composition to other listed heats difiering principally therefrom in that it does not. contain boron. The strengthening equivalency of the additions of tungsten, molybdenum, niobium and tantalum on an atomfor-atom basis is clearly apparent from these testing results.

From these and other testing results it was determined that a temperature of 2150 F. is adequate for the solution treatment and that the aging temperature should be about 1650 F. for best ductility with no sacrifice in strength. From the test data the following assessment has been made of several characteristics of my alloy.

COMPOSITION A comparison of the test results with the composition for the various heats of my alloy reveals that boron within fairly critical limits has a remarkable strengthening effect without causing a corresponding lowering of ductility and high temperature properties as would normally be expected. In the alloys of my invention the workability was not substantially impaired and the and 1000 hour 1500 F. stress rupture strength was improved by the boron addition to analogous compositions by as much as 6500 and 4000 p.s.i., respectively. In this regard, compare identically treated heats A and B in Tables I and II. Compare also identically treated heats A and E in Tables Iand II. It will also be noted from a comparison of these heats A and E and others of the series of analogous compositions comprising heats A, B, C, D, E, F and G that the optimum boron content appears to lie somewhere between 0.1 and 0.4 atomic percent in alloys of this composition and that the addition of as much as about 0.6 atomic percent causes a drastic reduction in properties as shown by heat G. It should, however, be noted that heat G is still substantially stronger 7 than the reference alloy. It is apparent further that a somewhat higher strength is realized in heats containing tantalum as shown in heats H, I, L, M, N and P, although in general, from the data it is equally apparent that tungsten, molybdenum, tantalum and niobium exert a sub stantially equivalent strengthening effect on an atom-foratom basis on my alloy. By and large, however, variations in the boron content for otherwise analogous compositions have a far greater effect upon my a lloys high temperature strength than similar variations in theother constituents. Y

STRESS RUPTURE The data obtained from the stress rupture test of alloy indicates that optimum stress rupture strengths are FATIGUE STRENGTH Only representative fatigue strength data has been presented in order to conserve space. The 1500 F. fatigue strength of these alloys ranges between about 40,000 p.s.i. and about 45,000 p.s.i. for 10 cycles. This compares to a 1500" F. fatigue strength of about 35,500 p.s.i. for 10 cycles for the reference alloy.

TENSILE PROPERTIES Inspection of the data presented in Table III reveals a general decline in both room temperature and high temperature tensile strength of the boron containing alloys compared to heat A. It will be noted, however, that while the ductility of heat A decreascswith an increase in temperature, the reverse is generally true of the boron beats, a highly desirable effect which would not normally be expected.

From the foregoing, it is readily apparent that a wrought alloy has been provided which is significantly stronger at temperatures of the order of 1650" F. than has been known before.

A further advantage inherent in thisalloy is the flexibility of its composition. Inasmuch as some or all of the elements of the group consisting of tungsten, molybdenum, niobium and tantalum may be classified as strategic materials and, therefore, their availability and cost are subject to controls other than the usual laws of economics, it is of importance that this allow may be made with a relatively wide choice of combinations of these elements providing the substitutions and combinations are made atom-for-atom and not weight-for-weight. This, of course, permits the manufacturer to select those ingredients available at the lowest cost of production. It should be further noted, particularly with regard to the titanium content, that the preferred vacuum melting technique provides an alloy having very low amounts of inclusions such as metallic oxides and nitrides. These impurities exist in my alloy in weight percentages of the order of 0.002 percent oxygen and 0.0005 percent nitrogen. As previously pointed out this is important in the maintenance of the full potential strengthening effect of the titanium content.

It is to be understood that small amounts of elements other than those specifically disclosed will inevitably be present in this or any other commercially produced alloy. However, these elements are present only as impurities and as such do not materially alter the composition of the alloy as claimed. While I have discussed my alloy as having particular utility in the manufacture of turbine buckets or blades for aircraft type turbojet engines, such description is merely illustrative and by way of example and I do not desire or intend that the alloy or wrought articles made therefrom be limited to such use.

What I claim as new and desire to secure by Letters Patent of the United States is: r

1. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 35 to 40 atomic percent cobalt; 25 to 30 atomic percent nickel; 18 to 25 atomic percent chromium; 1.5 to 5 atomic percent tungsten; up to 3.5 atomic percent molybdenum; up to 3.5 atomic percent tantalum; up to 1.5 atomic percent niobium; 4 to 7.5 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 0.01 to 0.4 atomic percent boron; and the remainder of said alloy being substantially all iron, not to eX'ceedTZ'atomic percent. l e

2. A metal alloy having high mechanical and physicochemical properties at elevated temperatures'consisting essentially of 35 to 40 atomic percent cobalt; 25 to 30 atomic percent nickel; 18 to 25 atomic percent chromium; 1.5 to 5 atomic percent tungsten; up to. 3.5 atomic percent molybdenum; 4 to 7.5 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 0.01 to 0.4 atomic percent boron; and the remainder of said alloy being substan tially all iron not to exceed 2 atomic percent.

3. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of about 38 atomic percent cobalt; 28 atomic percent nickel; 23 atomic percent chromium; 2.4 atomic percent tungsten; 2.4 atomic percent molybdenum; 4.8 atomic percent titanium; 1.2 atomic percent carbon; and 0.3 atomic percent boron. 3

4. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 35 to 40 atomic percent cobalt; 25 to 30 atomic percent nickel; 18 to 25 atomic percent chromium; 1.5 to 5 atomic percent tungsten; 0.6 to 1.0 atomic percent tantalum; 4 to 7.5 atomic percent titanium; 0.1 to 2 atomic percent carbon; 0.01 to 0.4 atomic percent boron; and the remainder of said alioy being substantially all iron not to exceed 2 atomic percent.

5. A metal alloy having high mechanical and physicachemical properties at elevated temperatures consisting essentially of about 37.6 atomic percent cobalt; 28.2 atomic percent nickel; 22.6 atomic percent chromium;

3.86 atomic percent tungsten; 0.83 atomic percent tantalum; 4-86 atomic percent titanium; 0.98 atomic percent carbon; 0.056 atomic percent boron; and the remainder of said alloy being substantially all iron.

6. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 35 to 40 atomic percent cobalt; 25 to 30 atomic percent nickel; 18 to 25 atomic percent chromium; 1.5 to 5 atomic percent tungsten; 1.5 to 3.5 atomic percent molybdenum; 0.8 to 1.5 atomic percent tantalum; 4.5 to 7.5 atomic percent titanium; 0.5 to 1 atomic percent carbon; 0.005 to 0.1 atomic percent boron; and the remainder of said alloy being substantially all iron not to exceed 2 atomic percent. 0

7. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of about 37 atomic percent cobalt; 27.6 atomic percent nickel; 22 atomic percent chromium; 1.9 atomic percent tungsten; 2.0 atomic percent molybdenum; 1.1 atomic percent tantalum; 6.0 atomic percent titanium; 0.7 atomic percent carbon; 0.012 atomic percent boron; and the remainder of said alloy being substantially all II'OIl.

8. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of 35 to 40 atomic percent cobalt; 25 to 30' atomic percent nickel; 18 to 25 atomic percent chromium; 1.5 to 5 atomic percent tungsten; 1.5 to 3.5 atomic per cent molybdenum; 0.8 to 1.5 atomic percent niobium; 4 to 6 atomic percent titanium; 0.5 to 1.5 atomic percent carbon; 0.011 to 0.1 atomic percent boron; and the balance 9 of said alloy being substantially all iron not to exceed 2 atomic percent.

9. A metal alloy having high mechanical and physicochemical properties at elevated temperatures consisting essentially of about 37 atomic percent cobalt; 28 atomic percent nickel; 22.5 atomic percent chromium; 2.0 atomic percent tungsten; 2.12 atomic percent molybdenum; 1.00 atomic percent niobium; 5.0 atomic percent titanium; 1.00 atomic percent carbon; 0.054 atomic percent boron;

and the remainder of said alloy being substantially a1 References Cited in the file of this: patent UNITED STATES PATENTS 

1. A METAL ALLOY HAVING HIGH MECHANICAL AND PHYSICOCHEMICAL PROPERTIES AT ELEVATED TEMPERATURES CONSISTING ESSENTIALLY OF 35 TO 40 ATOMIC PERCENT COBALT, 25 TO 30 ATOMIC PERCENT NICKEL, 18 TO 25 ATOMIC PERCENT CHROMIUM, 1.5 TO 5 ATOMIC PERCENT TUNGSTEN, UP TO 3.5 ATOMIC PERCENT MOLYBDENUM, UP TO 3.5 ATOMIC PERCENT TANTALUM, UP TO 1.5 ATOMIC PERCENT NIOBIUM, 4 TO 75 ATOMIC PERCENT TITANIUM, 0.5 TO 1.5 ATOMIC PERCENT CARBON, 0.01 TO 0.4 ATOMIC PERCENT BORON, AND THE REMAINDER OF SAID ALLOY BEING SUBSTANTIALLY ALL IRON, NOT TO EXCEED 2 ATOMIC PERCENT. 